(1) Field of the Invention
The present invention relates in general to mobile communication systems and particularly to high-frequency signal demodulation in a GSM telephony system.
(2) Description of Related Art
One standard for mobile communication is the Global System for Mobile communication (GSM). Each GSM network comprises a plurality of base transceiver stations or base cells that manage communication links with thousands of mobile units, typically hand-held mobile telephones. The base cells are connected to a controller that manages radio channel set-up and hand-off between base cells to facilitate roaming communication. The controller connects to a switching center, which in turn connects the call through to the local telephone network. Information is transmitted between the base station and the mobile unit by phase modulating the carrier signal so that the angle of the sine wave carrier deviates from the original angle by an amount proportional to the instantaneous value of the modulating wave. Accordingly, accurate detection of the carrier signal frequency is critical to ensuring clear and continuous reception.
Even though a mobile unit is camped on a base cell (often referred to as being attached to a serving cell), it must still monitor transmissions from neighboring cells. Specifically, as the mobile unit roams, it monitors signal strength from the serving cells and neighboring cells and must be able to demodulate transmissions from these cells to ensure continuous reception of transmitted information. If these cells are transmitting at frequencies that are each offset by xc2x150 Hertz relative to the target frequency and if the local oscillator of the mobile unit is offset by xc2x1100 Hertz, the combined frequency offset could be as large as xc2x1200 Hertz. Accordingly, the mobile unit must be able to correctly demodulate signals from a variety of transmitting base stations, each of which may be transmitting at a frequency that is offset from the expected or nominal frequency.
Further complicating the demodulation process is the fact that the mobile unit may freely move (and often at high rates of speed), so the transmitted signal may have a frequency shift due to the relative movement of the mobile unit either toward or away from the serving cell.
Reception problems become more apparent as the mobile unit roams so that distance to the serving cell and the neighboring cells rapidly changes. Indeed, if the movement is at a high rate, the Doppler effect can induce a significant frequency offset that will be manifested as lost data. For example, rapidly moving toward one cell and away from a second cell, the frequency of transmitted signals from the two cells may be offset by about xc2x1500 Hertz. Doppler errors can then combine with the previously identified errors to give composite frequency errors up to xc2x1700 Hertz.
One will appreciate that since frequency error is rotation of phase with respect to time, data will be lost or will require multiple attempts at transmitting the data. Frequency offset introduces demodulation errors because in GSM systems, signals are phase modulated with digital information encoded on the analog radio signal.
Thus correcting for frequency offset introduced by movement of the mobile unit and transmission at frequencies that deviate from the target frequency is critical in achieving substantially error-free demodulation of the transmitted signal. What is needed is an improved receiver unit in a mobile telephone system that is capable of correcting for a frequency offset of up to one thousand (xc2x11000) Hertz.
The present invention relates to a multiple-hypothesis frequency detection or demodulation procedure in a mobile communication system. More particularly, the present invention determines a nominal frequency for demodulation of a received signal.
In mobile communication systems such as the Global System for Mobile communication (GSM) system, data is transmitted as a digitally encoded burst phase modulated on an analog radio carrier signal. Since it is difficult to determine the actual frequency of the carrier signal, the present invention digitizes the transmitted signal and tests multiple hypotheses to determine the carrier frequency. The results of these tests provide an accurate estimate of the carrier frequency and this estimate is used for demodulation.
Specifically, in the present invention, the GSM mobile unit receives a transmitted signal from one or more base stations. A representation of the signal is filtered, digitized and stored in memory. The mobile unit recovers the encoded data by demodulating the digital representation by applying three demodulation hypotheses to at least the first two received signal bursts. The first hypothesis is based on an assumption that the transmitted signal was received without frequency offset or phase rotation. After demodulation, the recovered data is checked to determine if any errors are present. The mobile unit demodulates the digital results a second time applying a second hypothesis that the data burst has a phase rotation of +a degrees on the first half of the burst and a phase rotation of xe2x88x92a degrees on the last half of the burst. The recovered data is again checked for errors. Finally, the mobile unit demodulates the digital results applying a third hypothesis that the data burst has a phase rotation of xe2x88x92a degrees on the first half burst and a phase rotation of +a degrees on the second half burst. Again the recovered data is checked to determine if the demodulation was error free. Even though two or more hypotheses may provide recovered data that appears error free, the recovered data may differ. In such instances, the two versions of recovered data may further include a Euclidean distance measure test or other similar test to provide a basis for deciding which version of the recovered data is most correct.
An estimate of frequency offset is based on a comparison of the results of the three separate demodulation attempts. For example, if the demodulation attempts all agree, the signal was received with substantially no frequency offset. However, if either the second or third hypotheses properly recover the data while the first hypothesis generated errors, frequency offset approaches the selected frequency as used in the respective demodulation attempt. In this manner a substantially accurate estimate of the frequency offset is obtained in real time with little compute overhead and without any a priori frequency offset estimate. Based on the frequency offset estimate, the transmitted signal may be demodulated without adjustment of the mobile unit""s local oscillator.